The salicylate ion increases the rate of bile flow (choleretic effect) and bile salts are known to affect the colonic absorption of oxalate. Owing to this physiological relevance of salicylate and oxalate ions, critical micelle concentration (cmc) values of sodium deoxycholate (NaDC) were determined in aqueous sodium oxalate, sodium salicylate, and sodium chloride solutions by using surface tension, fluorescence, and EMF methods. The results indicate, besides a counterion effect, the influence of coanions on the cmc. In the range from 25 to 40 degrees C, cmc increases almost linearly with temperature. In the temperature range from 30 to 40 degrees C, the counterion binding constant beta of NaDC micelles has the same value (0.17 +/- 0.01) in the presence of sodium chloride and sodium salicylate. On the other hand, in sodium oxalate solution beta = 0.05 +/- 0.02 when oxalate concentration is less than or equal to c* and beta = 0.48 +/- 0.04 above c*, where c* approximate to 0.038 mol kg(-1). EMF measurements also supported this type of counterion binding to NaDC micelles in sodium oxalate solutions. In sodium oxalate solution, at c* a change in the shape of deoxycholate micelles is expected to take place. Salicylate, oxalate, and chloride coanions have a similar effect on the adsorption of NaDC. This study reveals that the choleretic effect of salicylate is not due to the influence of salicylate ions on the micellization of NaDC. (C) 2011 Elsevier Inc. All rights reserved.